Use of a distributed hash table to federate many small-sized IMS core infrastructures

ABSTRACT

Multimedia subsystem, IMS, containing a set of call session control function elements, CSCF, suitable for sending a signaling message (m) between a first communication client (T) connected to said multimedia subsystem and at least one second communication client, where this second communication client may be connected to said multimedia subsystem or remote and accessible through an interrogating functional element, I-CSCF, and routing methods used to determine a path for the transmission of the signaling message, based on a logical address contained in the signaling message. The multimedia subsystem, IMS, is characterized in that the routing methods implement a distributed hashing table.

The invention concerns communication networks and, more specifically, IPMultimedia Subsystems (IMS) as standardized by the 3GPP and the ETSITispan.

The IMS architecture is specified by the document TS 23.228 of the3GPP™, entitled “IP Multimedia Subsystem (IMS); Stage 2 (Release 7)”.

The objective of the IMS architecture is to provide a convergencesolution between the fixed and mobile infrastructures of thecommunication networks. This common infrastructure also allows thedeployment of known telecommunication services (call transfer, billing,video-conference, etc.) or services yet to come.

FIG. 1 shows a simplified IMS architecture. It should be noted that thisarchitecture is functional: the same functional element may therefore bedistributed over several network devices and, vice versa, the samenetwork device may carry out several IMS architecture functions.

A multimedia subsystem IMS consists mainly of three CSCF (Call SessionControl Function) functions:

-   -   The P-CSCF (Proxy CSCF) is the contact point of the        communication terminals (or clients) T in the IMS communication        network. One of its roles is to authenticate the terminal T and,        if necessary, inspect the signaling messages coming from this        terminal.    -   The S-CSCF (Serving CSCF) provides the interface between the        application servers AS, and is used to route signaling messages        within the IMS communication network, in collaboration with the        Home Subscriber Server HSS, coming from or intended for a client        connected to this subsystem.    -   The I-CSCF (Interrogating CSCF) is used to interface with other        IMS networks and to route signaling messages from one subsystem        to another.

When a communication terminal registers with the IMS communicationnetwork, it sends a registration signaling message to an S-CSCF1functional element through the P-CSCF1 functional element to which it isattached.

This signaling message allows the S-CSCF1 element to add informationwithin the HSS1 subscriber server, connecting the logical address of theuser of terminal T (in other words the subscriber to the IMS network)with the identifier of the S-CSCF1 element.

An SLF (Subscription Locator Function) functional element is alsoaffected and memorizes the association between the logical address ofthe user of terminal T and the HSS1 subscriber server which possessesits information.

Finally, the S-CSCF1 element retains the association between the logicaladdress and the physical address of terminal T.

The S-CSCF1 functional element is also connected to an applicationserver AS which is used to provide subscribers with differenttelecommunication services: billing, call waiting, re-directing ofcalls, announcements, audio and/or video conference, etc. The S-CSCF1functional element is therefore able to analyze the signaling messagesin order to contact one or more application servers AS depending on thecontent.

For an incoming call intended for the client T, the SLF, HSS1 andS-CSCF1 elements are used as a means for associating the logical addresscontained in the incoming signaling message and the physical address ofthe client T.

As specified by the 3GPP (3^(rd) Generation Partnership Project) andETSI (European Telecommunications Standards Institute) standardizationbodies, the communication protocol used for the transmission ofsignaling messages is SIP (Session Initiation Protocol). This protocolis specified in the RFC 3261 of the IETF (Internet Engineering TaskForce) and extended upon by subsequent RFCs.

The registration signaling message is therefore normally a SIP“REGISTER” message, and the invitation signaling message is a SIP“INVITE” message.

With the deployment of IMS architectures, the applicant expects anexplosion in logical addresses and terminals likely to be connected. Infact, it will become possible to connect a large number of terminals forthe same subscriber (computer, fixed line telephone, mobile telephone,personal assistant, television, etc.). It will also be possible toconnect devices which are currently usually disconnected fromcommunication networks (household appliances, etc.) in order to providenew services to users.

The subscriber server HSS and above all the subscription locationelement SLF will then increase considerably in size. They will becomebottleneck nodes within the IMS architecture: as they increase in size,they will become more difficult to manage and their access times willalso increase, at the expense of the general system performance.

One aim of the invention is to resolve this problem by proposing adistributed architecture.

A distributed approach has already been proposed by the article “Usingan External DHT as a SIP Location Service” by Kundan Singh and HenningSchulzrinne, or by the article “Peer-to-peer Internet Telephony usingSIP” by the same authors.

The patent request WO2006/068365 presents a similar approach.

This approach is illustrated in FIG. 2.

The terminals (or clients) T1, T2, T3 are connected to a DistributedHashing Table, or DHT. This distributed hashing table plays the role ofSIP registrar in a traditional SIP architecture: the associationsbetween logical subscriber addresses and physical terminal addresses arememorized in this distributed hashing table.

Each of the terminals T1, T2, T3 possesses a part of this hashing table.Using the mechanisms of the distributed hashing table, a terminal canobtain a physical address of a terminal based on the logical address ofthe subscriber it wishes to call.

As the registrar is distributed over multiple terminals, there is nolonger congestion within a central subscriber server as in the IMSarchitecture.

However, this approach is not compatible with the IMS architecture, asspecified by the 3GPP and ETSI TISPAN standards.

In fact, the terminals must be modified: they do not interact with aP-CSCF functional element through a standardized interface, but mustimplement a distributed hashing table DHT and implement a specificprotocol.

Another state of the art is provided by the Skype Internet telephonyproduct. The principle is mainly the same but the protocol used is aproprietary protocol and not the SIP protocol.

Since this is a proprietary solution, the terminals must include amodule provided by the Skype company, and as previously, they do notinterface with a P-CSCF functional element of the IMS architecture.

It is generally accepted that the distributed approach, based on theintelligence of the terminals, and the IMS approach, based on thelocation of the intelligence in the communication network, areantinomic. For an example, see the participative encyclopedia“Wikipedia” at the address: http://fr.wikipedia.org/wiki/P2P

The aim of this invention is to adopt the principle of a distributedarchitecture to implement the association between logical addresses andphysical addresses, but remaining compatible with the IMS architecture.In particular, one of the aims of the invention is to be transparentwith regard to the communication terminals.

To do this, the first aim of the invention is a multimedia subsystem,IMS, containing a set of call session control function elements, CSCF,suitable for sending a signaling message between a first communicationclient connected to said multimedia subsystem and at least one secondcommunication client. The second communication client may be connectedto the multimedia subsystem or remote and accessible through aninterrogating functional element, I-CSCF. The multimedia subsystem alsocontains routing methods used to determine a path for the transmissionof the signaling message, based on a logical address contained in thesignaling message. The multimedia subsystem is characterized in thatthese routing methods implement a distributed hashing table.

According to a first execution method of the invention, the routingmethods contain a subscriber selection functional element, SLF, used todetermine the subscriber server HSS associated with the logical addressand implemented by a distributed hashing table.

According to a second execution method, the multimedia subsystem, IMS,contains multiple domains, each containing a set of call session controlfunction elements, CSCF. The routing methods contain a federatingfunctional element used to determine one of these domains based on thelogical address (contained in the signaling message), and areimplemented by a distributed hashing table.

According to an implementation of this execution method of theinvention, the serving functional elements, S-CSCF, of at least some ofthe domains form the distributed hashing table nodes.

At least one domain can be planned so that signaling messages from saidfirst communication client connected to this domain are sent to theinterrogating functional element, I-CSCF, of the domain concerned. Thisinterrogating functional element is then itself intended to interrogatethe federating functional element in order to determine the path for thesignaling messages.

The interrogating functional element can for example implement thefunctions of a THIG gateway.

The interrogating functional element, I-CSCF, can also be intended, onreceipt of a signaling message from the first communication client, todetermine whether the recipient communication client determined by thelogical address contained in the signaling message is connected to oneof these domains, and if not, to send the signaling message outside themultimedia subsystem without interrogating the federating functionalelement.

The interrogating functional elements, I-CSCF, can on the other hand beintended, on receipt of a signaling message from the first communicationclient, to systematically interrogate the federating functional element(FF) and, in the absence of a response from this element determining apath, to send the signaling message outside the multimedia subsystem.

The interface between the interrogating functional elements of thedomains and the federating functional element can be a Dx interface,implementing the Diameter protocol.

This interface may contain an initial function used to insert anassociation between the logical address and the domain, and a secondfunction used to determine the domain corresponding to a given logicaladdress.

The logical addresses can contain an identifier corresponding to themultimedia subsystem.

The second aim of the invention is a communication network containing atleast one multimedia subsystem, IMS, as described previously.

The third aim of the invention is a federating functional element for amultimedia subsystem, IMS. The multimedia subsystem contains multipledomains, each containing a set of call session control functionelements, CSCF. According to this aim of the invention, the federatingfunctional element contains means to determine a domain based on alogical address contained in a signaling message, these meansimplementing a distributed hashing table.

The serving functional elements, S-CSCF, of at least some of thesedomains can form nodes of the distributed hashing table.

The federating functional element may have a Dx interface implementingthe Diameter protocol.

This interface may contain an initial function used to insert anassociation between the logical address and domain, and a secondfunction used to determine the domain corresponding to a given logicaladdress.

The fourth aim of the invention is a method for sending a signalingmessage through a multimedia subsystem, IMS, containing a set of callsession control function elements, CSCF, between a first communicationclient, connected to this multimedia subsystem, and a secondcommunication client which may be connected to the multimedia subsystemor remote and accessible through an interrogating functional element,I-CSCF.

The method includes a stage for the determination of a path for sendingthe signaling message, based on a logical address contained in thissignaling message.

The method is characterized in that this stage implements a distributedhashing table.

According to a first execution method, the method contains a stage forthe determination of a subscriber server HSS associated with the logicaladdress; this determination is carried out by a subscriber selectionfunctional element, SLF, implemented by a distributed hashing table.

According to a second execution method, the multimedia subsystemcontains multiple domains, each containing a set of call session controlfunction elements, CSCF.

The method contains a stage for the determination of a domain amongthese multiple domains, based on the logical address; the determinationis carried out by a federating functional element implemented by adistributed hashing table.

According to an implementation of this execution method, the servingfunctional elements, S-CSCF, of at least some of the domains form thenodes of the distributed hashing table.

Furthermore, at least one domain can be planned so that the signalingmessages from a communication client connected to this domain are sentto the interrogating functional element, I-CSCF, of the domain. Theinterrogating functional element can itself be intended to interrogatethe federating functional element in order to determine the path forthese signaling messages.

The invention also has the aim of a software product implementing thismethod in the different possible implementations.

The invention also has the aim of a system containing a domain nameserver, DNS, and a federating functional element according to the thirdaim of the invention.

Lastly, the invention has the aim of an interrogating functional elementI-CSCF for the domain of a multimedia subsystem, IMS, containingmultiple domains, each containing a set of call session control functionelements, CSCF, and a federating functional element used to routesignaling messages between these domains of the multimedia subsystem.

The interrogating functional element is intended to receive a signalingmessage from a first client connected to this domain and intended for asecond client.

It is characterized in that it is also intended to:

-   -   Determine whether the second client is connected to one of the        domains of the multimedia subsystem,    -   If this is not the case, to send the signaling message outside        the multimedia subsystem without interrogating the federating        functional element, and    -   If this is the case, to interrogate the federating functional        element in order to determine the path for this signaling        message.

The invention, its different implementations and its advantages willappear more clearly in the implementation description which follows,together with the attached figures.

FIG. 1, mentioned previously, represents the IMS architecture.

FIG. 2, also mentioned previously, represents a solution for the stateof the art.

FIG. 3 shows a diagram for a first implementation of the invention.

FIG. 4 shows a diagram for a second implementation of the invention.

FIG. 5 illustrates the operation of a distributed hashing table.

FIRST IMPLEMENTATION

According to a first implementation of the invention, the subscriptionlocation functional element, SLF, is implemented by a distributedhashing table.

The SLF (Subscriber Locator Function or Subscription Locator Function)functional element is a used to determine a subscriber server HSS in amultimedia subsystem IMS which contains several.

This functional element is for example described in the documents TS29.228 and TS 29.229, respectively entitled “IP Multimedia (IM)Subsystem Cx and Dx Interfaces; Signalling flows and message contents”and “Cx and Dx interfaces based on the Diameter protocol; Protocoldetails”. These two documents are produced by the 3GPP standardizationbody and are available on its website.

The SLF functional element is also mentioned in the RFC 4457 of the IETFentitled “The Session Initiation Protocol (SIP)—P-User-DatabasePrivate-Header (P-Header)”, published in April 2006.

This subscriber selection functional element SLF forms part of therouting methods of the multimedia subsystem IMS, in other words of allthe technical means used for the resolution of a logical addresscontained in a signaling message to route it towards the correspondingdestination. Other than this SLF element, the routing methods can alsoinclude subscriber servers HSS, the serving functional elements S-CSCF,and the domain name servers DNS.

It is possible that one multimedia subsystem IMS may have severalsubscriber servers HSS. On FIG. 3, three subscriber servers HSS₁, HSS₂and HSS₃, have been shown.

When the interrogating functional element I-CSCF receives an incomingsignaling message (or an incoming call), it must determine how to routethis message to the recipient communication client. This is the case,for example, for SIP “Invite” messages aiming to set up a call session.These “Invite” messages, in accordance with the SIP protocol, containthe logical address of one or more recipient subscribers.

To route the “Invite” signaling message to the recipient communicationclient T, it interrogates the subscriber location functional elementSLF, providing in its request the logical address of the recipientcontained in the “Invite” signaling message.

This determines a subscriber server identifier associated with thislogical address, and sends it in a response message to the interrogatingfunctional element I-CSCF.

Possessing this identifier (its IP address, for example), it can theninterrogate the correct subscriber server HSS1, in other words the onewhich has the information relating to the user subscriber of thecommunication client T and corresponding to the logical address.

The subscriber server contains associations between logical addressesand identifiers of serving functional elements S-CSCF. By providing thelogical address in its request, the interrogating functional elementI-CSCF receives in response an identifier of the serving functionalelement S-CSCF₁ with which the recipient communication client T isregistered.

It can then send the “Invite” signaling message to this servingfunctional element S-CSCF₁. This has the information relating to thecommunication client T and to the user subscriber of this client.

It in particular has the association between the logical address of thesubscriber and the physical address of the communication client T. Itcan then send the signaling message to the recipient subscriber usingthe physical address (for example its IP address) of its communicationclient T.

Furthermore, when a communication client T registers with the multimediasubsystem IMS, information is memorized in the serving functionalelement S-CSCF₁ to which it is attached, in a subscriber server HSS₁ andin the subscriber selection functional element SLF.

It is possible to limit the size of the subscriber servers HSS byscaling them down, but the subscriber location functional element SLFremains a centralized element, the size of which is proportional to thenumber of subscribers (in other words registered logical addresses).

According to the invention, this subscriber location functional elementSLF is implemented by a distributed hashing table. It therefore containsa set of nodes N₁, N₂ . . . N_(n) inter-connected between themselves.

The operation of this network of nodes will be explained in more detailbelow. It is important however that it should have the following as aninterface:

-   -   a function used to determine a subscriber server HSS based on a        logical address; and,    -   a function used to memorize in the distributed hashing table an        association between the logical address of the user of the        communication client T and the subscriber server HSS₁ which has        been assigned to it.

This first execution method has the advantage of only modifying theimplementation of the subscriber location functional element SLF.

This can retain the same interface and for example a Dx interfacecompliant with the Diameter protocol. In this way, it can interface withthe other functional elements of the multimedia subsystem IMS, withoutrequiring modification of the other elements.

SECOND IMPLEMENTATION

According to a second implementation of the invention, illustrated byFIG. 4, the multimedia subsystem IMS contains multiple domains, IMS₁,IMS₂ . . . IMS_(n). Each domain can be seen as an entirely separate IMSsubsystem, having its own call session control function (CSCF) elements:

-   -   a serving functional element S-CSCF, respectively S₁, S₂ . . .        S_(n)    -   an interrogating functional element I-CSCF, respectively I₁, I₂        . . . I_(n).    -   (if necessary) a proximity functional element P-CSCF, not shown.

The routing methods contain, as in the previous example, subscriberselection functional elements SLF₁, SLF₂ . . . SLF_(n); servingfunctional elements S₁, S₂ . . . S_(n); a domain name server DNS;subscriber servers (HSS, not shown).

According to the invention, they also contain a federating functionalelement FF. This new functional element is used to determine one of thedomains IMS₁, IMS₂ . . . IMS_(n), based on a logical address containedin a signaling message.

This federating functional element FF is implemented by a distributedhashing table.

Distributed Hashing Table

A distributed hashing table (DHT) is a technology used to memorize andretrieve information in a widely distributed system such as a peer topeer network. In principle, the content of the hashing table isdistributed over all the stations or nodes of the network and there isno centralized device.

Such distributed hashing tables are described for example in the article“Looking up Data in P2P Systems” by H. Balakrishna, F. Kaashoek, D.Karger, R. Morris and I. Stoica, published in February 2003, in thereview Communications of the ACM, vol. 46, no. 2.

The patent request WO2006/068365 mentions some of the principles of thestate of the art and proposes a new distributed hashing table approach.

There are different possible implementations of a distributed hashingtable. FIG. 5 shows a simplified example to help understanding, but theinvention should not be considered to be limited to this specificimplementation.

The set of nodes X₁, X₂, X₃ . . . X_(N) of the distributed hashing tableDHT plays the same role. In order to memorize or retrieve a value fromthis table (or network), any of these nodes can be consulted.

In the example of FIG. 5, the interrogating functional element I-CSCFinterrogates the node X₃, providing it with a logical address.

This then calculates a hashing function on this logical address.

A hashing function is a function used to convert a value belonging to alarge set (all possible logical addresses) to a second value belongingto a reduced set. This second value here is an integer between 1 and N,where N is the number of nodes in the distributed hashing table DHT.

The hashing function H should generally respect the followingmathematical property: H(x)≠H(y)

x≠y

Furthermore, if x=y then the probability must be greater/less thanH(x)=H(y).

The value i returned by the hashing function indicates to node X₃ thatnode X_(i) is probably the one which possesses the association betweenthe logical address received and the corresponding domain. It sends therequest to this node X_(i). If this node does indeed possess theassociation, it can return the domain associated with the logicaladdress. Otherwise, it can send the request to another node (neighbor)which may possess the association.

Iteratively, the algorithm converges to the node which possesses theassociation.

The principle for the memorization of a new association is similar. Anynode of the DHT peer to peer network receives a request for memorizationand calculates a value based on the hashing function and the logicaladdress.

This value determines a specific node of the set of nodes of thedistributed hashing table DHT, to which the first node resends therequest for memorization. The node designated in this way memorizes theassociation.

According to one execution method of the invention, the nodes of thisdistributed hashing table are made up of all or some of the servingfunctional elements S₁, S₂ . . . S_(n).

This execution method is used in order not to add a new element withinthe global architecture, and to use the already defined functionalelements. It allows better respect for the current standardization ofthe multimedia subnetworks IMS.

Preferentially, the federating functional element FF presents a Dxinterface which can be used by the other functional elements (serving,S-CSCF, or interrogating, I-CSCF).

This Dx interface can typically implement the Diameter protocol,standardized by the IETF. Two main functions are of use for thisinterface: a function used to memorize a new association, and a functionused to retrieve the value associated with a value provided.

Registration of a New Client with a Domain

When a communication client T registers with a domain of the IMScommunication subsystem, information is memorized in the distributedhashing table of the federating functional element FF. This informationcontains the association between the logical address of the user of thiscommunication client T and an identifier for this domain.

Within the domain itself, the communication client is registered in aknown manner and as described previously.

Incoming Call

During an incoming call, a signaling message m is sent to the multimediasubsystem IMS. This signaling message m is typically an “Invite” messagecompliant with the SIP protocol. It contains the logical address of acalled subscriber. It is assumed in the example of FIG. 4 that thissubscriber is the user of the communication client T, connected to thedomain IMS₂.

This address, in accordance with the specifications of the 3GPP and theETSI TISPAN, is a SIP URI (Universal Resource Identifier).

It can take the form a@x.com. The term “a” identifies the subscriber,while the term “x.com” identifies the multimedia subsystem, IMS.

The association of this term “x.com” with the multimedia subsystem IMSis carried out by the domain name server DNS. It is used to determine aphysical address (an IP address, for example) of an entry point to thismultimedia subsystem.

Typically, it is intended to take into account service registrationsknown as “SRV registration”, defined by the RFC 2782 of the IETF.

Since the multimedia subsystem according to the invention does not havea unique entry point, all or part of the interrogating functionalelements, I₁, I₂ . . . I_(n), can be associated with the term “x.com”within the DNS server. This can choose which address to send in a randommanner, or according to a heuristic method based on certain criteriasuch as the load of the different interrogating functional elements,priorities, etc.

In the example of FIG. 4, it is assumed that the domain name server DNShas determined the interrogating functional element I₁. The signalingmessage m is consequently sent to this functional element.

This then interrogates the federating functional element FF.

In its request, it inserts the logical address of the called user (here,“a@x.com”).

In response it receives the identifier of the domain with which theterminal used by the called party is connected. This identifier can forexample be the physical address (IP) of the interface functional elementof the domain in question.

In the example of FIG. 4, this identifier is that of the domain IMS₂,and the physical address sent can be that of the interface functionalelement I₂.

The interrogating functional element I₁, can then send the signalingmessage m to the interrogating functional element I₂.

The interrogating functional element I₂ can then act in accordance withthe standardization of the IMS, since the domain IMS₂ is a standardmultimedia subsystem in itself. The signaling message m can then berouted to the terminal T, used by the called party.

Another situation may arise corresponding to the case in which theterminal T is connected to the domain IMS₁. In this situation, theinterrogating functional element I₁ after interrogation of thefederating functional element FF sends the signaling message m directlyto its domain IMS₁ (more precisely to the serving functional elementS₁).

Outgoing Call

Returning to the example of FIG. 4, it is assumed now that thecommunication client (or terminal) T, connected to the domain IMS₂,sends a signaling message representing an outgoing call.

The description in fact only applies to the case of an outgoing callfrom the domain IMS₂. The situation in which the called communicationclient is connected to the same multimedia subsystem IMS₂ corresponds tothe state of the art in terms of operation of a multimedia subsystem.

In the case of an outgoing call, the signaling message is sent throughthe domain or multimedia subsystem IMS₂ to the serving functionalelement S₂ (if necessary using a proximity functional element P-CSCF,not shown).

This serving functional element S₂ can then be intended to interrogatethe federating functional element FF in order to determine a path (orroute) for this signaling message.

Two scenarios may then arise: the called party may be connected to oneof the domains IMS₁ . . . IMS_(n), or to another network or multimediasubsystem.

As in the situation of an incoming call, described previously, thefederating functional element FF can determine to which domain thecalled communication client is connected.

According to a first implementation, the serving functional element S₂systematically interrogates the federating functional element FF. If thecalled party is connected to one of the other domains, the federatingfunctional element determines this and sends an identifier of thedetermined domain. If the called party is not connected to one of thesedomains IMS₁ . . . IMS_(n), the federating functional element FF doesnot find its address and sends a specific message which can beinterpreted by the serving functional element S₂ as meaning that thecalled communication client is outside the domains IMS₁ . . . IMS_(n).This specific message can be an error message or a message containing aspecific identifier to indicate the absence of the called communicationclient in its information base.

It is also possible to plan for the serving functional elements to havemeans to determine whether or not the communication client forms part ofone of the domains IMS₁ . . . IMS_(n) without having to interrogate thefederating functional element FF.

According to another execution method of the invention, the domain isconfigured in such a way that all the outgoing signaling messages aresent to the interrogating functional element I-CSCF.

This execution method can for example be implemented by granting theinterrogating functional element I-CSCF the functions of a THIG(Topology Hiding Internetwork Gateway). The interrogating functionalelement I-CSCF then acts as the only interface between the IMSsub-system and the external communication networks. It is also used tomask the internal topology of the IMS with regard to these externalnetworks.

The features of a THIG gateway are specified in the documents TS 24.228and TS 23.228 of the 3GPP.

In this execution method, it is the interface functional elements whichare intended to interrogate the federating functional element.

This interrogation can be systematic: On receipt of the signalingmessage from the communication client T, the interrogating functionalelement I₂ interrogates the federating functional element FF. Thisresponds either with an identifier of the domain to which the calledclient is connected, or with a value indicating that the called clientis not connected to any of the domains IMS₁ . . . IMS_(n).

As mentioned in the previous execution method, the interrogatingfunctional element I₂ is intended, on receipt of the signaling message,to determine whether the called communication client (i.e. recipient ofthe signaling message) is connected to one of the domains IMS₁ . . .IMS_(n). These means are independent of the federating functionalelement, and used to reduce the communications caused by the routing ofa signaling message.

To do this, the interrogating functional element I₂ can contain a tablememorizing all of the communication clients connected to one of thedomains.

Alternatively, the determination can be based on a name convention. Thelogical address of the communication client can depend on the globalmultimedia subsystem. For example, the address a@x.com means that theterminal “a” is connected to the global domain determined by the domainname “x”. With such a convention, it is easy to determine through theinterrogating functional elements whether or not this name correspondsto its own name.

If it does not, it means that the client belongs to another network andthe signaling message must be routed externally. If the namecorresponds, however, it means that the federating functional elementmust be interrogated and the signaling message must be routed to thedomain corresponding to the response given by this federating functionalelement FF.

This operating method in which all the outgoing signaling messages passthrough the interrogating functional element (“THIG” method) has theadvantage of only requiring the modification of the interrogatingfunctional elements. In the situation of an incoming call or that of anoutgoing call, it is these elements which are responsible forinterrogating the federating functional element FF, and therefore onlythey need to be intended for this purpose and to process the responsefrom the federating functional element FF.

The other functional elements, in particular the serving functionalelements, are unchanged.

Such an operating method therefore reduces the impact of this newfeature on the global architecture of the communication network, and thecost of its addition.

In all cases, the interrogating functional element (“THIG method”) orthe serving functional element (other execution method) are intended todetermine whether the recipient communication client is connected to oneof the domains (whether by its own means, or by interrogation of thefederating functional element FF).

If it is not, it sends the signaling message outside the network.Otherwise, it sends it to the domain corresponding to the identifiersent by the federating functional element FF.

This invention is therefore used to federate a smaller set of multimediasubsystems (the domains) to form a larger one.

The global multimedia subsystem IMS can in fact have an interface withregard to the other communication networks which masks itsimplementation and the fact that it contains multiple domains IMS₁, IMS₂. . . IMS_(n). The federating functional element is therefore used tomake the architecture of the multimedia subsystem and its breakdown intomultiple domains transparent for the signaling message senders and forthe rest of the communication networks.

It then becomes possible to open certain domains—of a reduced size—tomanagement by private operators. For example, a domain can represent acompany or a subsidiary of a company and its management can be made theresponsibility of an administrator of this company or this subsidiary.

The global multimedia subsystem, federating several domains, can beadministered by an operator which guarantees the interface with regardto the external network.

It should also be noted that the two implementations can be combined, sothat the subscriber selection functional elements (SLF) contained ineach domain IMS₁, IMS₂ . . . IMS_(n) are implemented by a distributedhashing table, and that these domains are federated by a federatingfunctional element FF.

From an architectural point of view the federating functional element FFcan be integrated with other functional elements. In particular, a blockcontaining the federating functional element FF and the domain nameserver DNS can be integrated into a single function, forming a system.

The invention claimed is:
 1. Multimedia subsystem, IMS, comprising:multiple domains, each domain containing a set of call session controlfunction elements, CSCF, configured to send a signaling message (m)between a first communication client (T) connected to said multimediasubsystem and at least one second communication client, a federatingfunctional element (FF) implemented by a distributed hashing table, thefederating functional element being configured to, perform a hashingfunction on a logical address of the at least one second communicationclient to determine a hash value, and route the signaling message to anyone of the multiple domains based on the hash value, wherein, said atleast one second communication client is connected to said multimediasubsystem or remote and accessible through an interrogating functionalelement, I-CSCF, contained in each domain, and the I-CSCF in each domainis configured to send the signaling message to an I-CSCF of aneighboring domain based on a physical address of an I-CSCF of a domainin which the second communication client is connected.
 2. Multimediasubsystem, IMS, according to claim 1, in which serving functionalelements, S-CSCF, of at least some of said domains form the nodes ofsaid distributed hashing table.
 3. Multimedia subsystem, IMS, accordingto claim 1, in which at least one domain is intended so that thesignaling messages from said first communication client connected tosaid domain are sent to the interrogating functional element, I-CSCF, ofsaid domain, said interrogating functional element itself being intendedto interrogate said federating functional element (FF) in order todetermine the path for said signaling messages.
 4. Multimedia subsystem,IMS, according to claim 3, in which said interrogating functionalelement implements the functions of a THIG gateway.
 5. Multimediasubsystem, IMS, according to claim 3, in which said interrogatingfunctional element, I-CSCF, is configured, on receipt of a signalingmessage from said first communication client, to determine whether therecipient communication client determined by the logical addresscontained in said signaling message is connected to one of said domains,and if not, to send said signaling message outside said multimediasubsystem without interrogating said federating functional element (FF).6. Multimedia subsystem, IMS, according to claim 3, in which saidinterrogating functional elements, I-CSCF, are configured, on receipt ofa signaling message from said first communication client, tosystematically interrogate said federating functional element (FF) and,in the absence of a response from this element determining a path, tosend said signaling message outside said multimedia subsystem. 7.Multimedia subsystem, IMS, according to claim 1, in which the interfacebetween the interrogating functional elements of said domains and saidfederating functional element (FF) is a Dx interface, implementing theDiameter protocol.
 8. Multimedia subsystem, IMS, according to claim 1,in which the interface between the interrogating functional elements ofsaid domains and said federating functional element (FF) contains afirst function used to insert an association between the logical addressand domain and a second function used to determine the domaincorresponding to a given logical address.
 9. Multimedia subsystem, IMS,according to claim 1, in which said logical addresses contain anidentifier corresponding to said multimedia subsystem.
 10. Communicationnetwork containing at least one multimedia subsystem according toclaim
 1. 11. Interrogating functional element, I-CSCF, for a domain of amultimedia subsystem, IMS, containing multiple domains, each domaincontaining a set of call session control function elements, CSCF,configured to send a signaling message (m) between a first communicationclient (T) connected to said multimedia subsystem and at least onesecond communication client, and a federating functional element (FF)implemented by a distributed hashing table, the federating functionalelement being configured to, perform a hashing function on a logicaladdress of the at least one second communication client to determine ahash value, and route the signaling message to any one of the multipledomains based on the hash value, said interrogating functional elementbeing configured to, receive the signaling message from the firstcommunication client, determine whether said at least one second clientis connected to one of said domains of said multimedia subsystem, ifthis is not the case, send said signaling message outside saidmultimedia subsystem without interrogating said federating functionalelement (FF), and if this is the case, interrogate said federatingfunctional element (FF) in order to determine the path for saidsignaling message, and send the signaling message to an I-CSCF of aneighboring domain based on a physical address of an I-CSCF of a domainin which the second communication client is connected.
 12. Interrogatingfunctional element according to claim 11, in which the interface withsaid federating functional element (FF) is a Dx interface, implementingthe Diameter protocol.
 13. Interrogating functional element according toclaim 11, in which said determination is carried out based on a logicaladdress contained in said signaling message.
 14. Interrogatingfunctional element according to claim 13, in which the interface withsaid federating functional element (FF) contains a first function usedto insert an association between the logical address and domain and asecond function used to determine the domain corresponding to a givenlogical address.
 15. Interrogating functional element, according toclaim 13, in which said logical addresses contain an identifiercorresponding to said multimedia subsystem.
 16. Method of sending asignaling message through a multimedia subsystem, IMS, containingmultiple domains, each domain containing a set of call session controlfunction elements, CSCF, between a first communication client, connectedto said multimedia subsystem, and a second communication client, whereinsaid second communication client is connected to said multimediasubsystem or remote and accessible through an interrogating functionalelement, I-CSCF, contained in each domain, said method including,performing a hashing function on a logical address of the secondcommunication client to determine a hash value, routing the signalingmessage to any one of the multiple domains based on the hash value, andsending, by an I-CSCF of a first domain, the signaling message to anI-CSCF of a second domain based on a physical address of the I-CSCF ofthe second domain in which the second communication client is connected.17. Method of sending a signaling message, according to claim 16, inwhich the serving functional elements, S-CSCF, of at least some of saiddomains form the nodes of said distributed hashing table.
 18. Method ofsending a signaling message, according to claim 16, in which at leastone domain is intended so that the signaling messages from acommunication client connected to said domain are sent to theinterrogating functional element, I-CSCF, of said domain, saidinterrogating functional element itself being intended to interrogatesaid federating functional element (FF) in order to determine the pathfor said signaling messages.
 19. A non-transitory computer-readablemedium implementing the method according to claim 16 when it is run on adata processing device.